1. Field of the Invention
The present invention relates to a six degree-of-freedom (DOF) motion measuring apparatus, and more particularly, to a swing arm type optical system using the 6-DOF measuring apparatus to measure the motion of a slider in a hard disk drive (HDD).
2. Description of the Related Art
The position and orientation of an object (rigid body) in 3-dimensional (3-D) space can be measured by a variety of methods. As one of the methods extensively used, the position of an object is expressed as position vector in an orthogonal coordinate system, and the orientation of the object is expressed using Euler angles. The Euler angles are angles of rotation of an object about x, y, and z axes of the reference coordinate system, are referred to as rolling, pitching, and yawing angles, and are denoted by xcex3, xcex2, and xcex1, respectively.
FIG. 1 illustrates the concept of 6-DOF motion and symbols used for describing the motion. As shown in FIG. 1, coordinate system Ow is a reference coordinate system used to express motion of an object 1. Coordinate systems Ow, and Os are defined on object 1. Coordinate system Os is fixed to and moves along with object 1. Coordinate system Ow, has the same orientation as reference coordinate system Ow and the same origin as coordinate system Os. The position of object 1 in coordinate system Os, is expressed by position vector {right arrow over (t)}w=[tx ty tz]T. Tsw is a matrix having elements which include the parameters tx, ty, tz, xcex3, xcex2, and xcex1, as below, and Tsw defines the position and orientation of object 1 in coordinate system Os with respect to the reference coordinate system Ow:                               T          s          w                =                  [                                                                      c                  ⁢                                      xe2x80x83                                    ⁢                  α                  ⁢                                      xe2x80x83                                    ⁢                  c                  ⁢                                      xe2x80x83                                    ⁢                  β                                                                                                  c                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    β                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    γ                                    -                                      s                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    c                    ⁢                                          xe2x80x83                                        ⁢                    γ                                                                                                                    c                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    β                    ⁢                                          xe2x80x83                                        ⁢                    c                    ⁢                                          xe2x80x83                                        ⁢                    γ                                    +                                      s                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    γ                                                                                                t                  x                                                                                                      s                  ⁢                                      xe2x80x83                                    ⁢                  α                  ⁢                                      xe2x80x83                                    ⁢                  c                  ⁢                                      xe2x80x83                                    ⁢                  β                                                                                                  s                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    β                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    γ                                    +                                      c                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    c                    ⁢                                          xe2x80x83                                        ⁢                    γ                                                                                                                    s                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    β                    ⁢                                          xe2x80x83                                        ⁢                    c                    ⁢                                          xe2x80x83                                        ⁢                    γ                                    -                                      c                    ⁢                                          xe2x80x83                                        ⁢                    α                    ⁢                                          xe2x80x83                                        ⁢                    s                    ⁢                                          xe2x80x83                                        ⁢                    γ                                                                                                t                  y                                                                                                                          -                    s                                    ⁢                                      xe2x80x83                                    ⁢                  β                                                                              c                  ⁢                                      xe2x80x83                                    ⁢                  β                  ⁢                                      xe2x80x83                                    ⁢                  s                  ⁢                                      xe2x80x83                                    ⁢                  λ                                                                              c                  ⁢                                      xe2x80x83                                    ⁢                  β                  ⁢                                      xe2x80x83                                    ⁢                  c                  ⁢                                      xe2x80x83                                    ⁢                  γ                                                                              t                  z                                                                                    0                                            0                                            0                                            1                                              ]                                    (        1        )            
where c and s denote cosine and sine, respectively.
The coordinate system Os is fixed to object 1, and the position and orientation of object 1 are expressed using Tsw. To calculate the values of the six elements tx, ty, tz, xcex3, xcex2, and xcex1 is to measure the position and orientation of object 1 in 3-D space.
According to conventional methods used to measure the position and orientation of object 1, multiple degree-of-freedom displacement is measured using sensors mounted on each axis of coordinate system.
FIG. 2 illustrates the concept of measuring the coordinates and orientation of an object in a 2-D plane using conventional capacitance-type proximity sensors. As shown in FIG. 2, signals from x1 and y1 proximity sensors 21 and 25 are used to measure displacement in x- and y-axial directions. An x2 proximity sensor 23 is installed parallel to the x1 proximity sensor 21 to measure the angle of rotation. However, to measure 6-DOF motion in 3-D space, two proximity sensors are required for each direction. Thus, to measure 6-DOF displacement using the conventional method, a plurality sensors are needed for each axis, which causes many difficulties in actual applications. Also, when such capacitance-type proximity sensors are used, the material of object 1 to be measured is limited to metal. In addition, installation of the sensors may be difficult depending on the shape of object 1. A small space must be maintained between object 1 and the proximity sensors 21, 23, and 25.
On the other hand, a Mikelson interferometer can be used as an apparatus for measuring 6-DOF motion of an object. FIG. 3 illustrates the structure of a conventional Mikelson interferometer applied to measure one-dimensional displacement. As shown in FIG. 3, a laser source 30, a beam splitter 32, and a cube corner reflector 34 are fixed in position, and another cube corner reflector 36 is affixed to the surface of object 1 whose motion is to be measured, so that optical paths are formed, as shown in FIG. 3. This complex configuration is for measuring one-dimensional displacement, and six such interferometers must be used to measure 6-DOF displacement. In addition to a configuration of six interferometers being significantly complicated, it is difficult to keep the optical path of each interferometer aligned for 6-DOF displacement.
FIG. 4 illustrates the concept of measuring 6-DOF motion of an object by conventional four position-sensitive detectors (PSDs). The 6-DOF displacement measuring system of FIG. 4, which is suggested in an article in Optical Engineering, Vol. 36, No. 8, pp. 2287-2293 (1997), includes four beam splitters 45, 46, 47, and 48, which are mounted on an object 1 whose motion is to be measured, four PSDs 41, 42, 43, and 44, and two lenses 49a and 49b. Transitions and rotations in three axial directions of the object 1 are measured by this system with a resolution of 0.05 xcexcm and 0.25 xcexcrad, respectively. The 6-DOF measuring system is advantageous in that 6-DOF transitional and rotational motions are simultaneously measured. However, the object 1 should be large enough such that four beam splitters 45, 46, 47, and 48 can be mounted thereon, and the 6-DOF measuring system is unsuitable for measuring high-speed motion.
FIG. 5 illustrates the concept of measuring 6-DOF displacement using a conventional apparatus in which a photodetector assembly is affixed to an object whose position and orientation are to be measured. The 6-DOF displacement measuring apparatus of FIG. 5 is disclosed in U.S. Pat. No. 5,884,239 by Romanik. As shown in FIG. 5, vertical and horizontal planar laser beams 56 are emitted from a scanner 56. The vertical planar laser beam scans in the horizontal direction and the horizontal planar laser beam scans in the vertical direction, so that a particular area within which the position and orientation of an object is to be measured is scanned with the laser beams. Four photodetectors 51, 52, 53, and 54 are given a particular 3-D arrangement defining a shape. As this photodetector assembly is scanned with the vertical and horizontal planar laser beams, each of the photodetectors 51, 52, 53, and 54 irradiated with the laser beams detects the intensity of the laser beams. The photodetectors 51, 52, 53, and 54 detect the laser beams in a particular order according to the shape, position, and orientation of the photodetector assembly. Since the shape of the photodetector assembly is constant, the position and orientation of the photodetector assembly can be measured by measuring the timing of detecting laser beams by each of the photodetectors 51, 52, 53, and 54. Based on this principle, the position and orientation of an object (not shown) can be measured by mounting such a photodetector assembly on the object. A single external photodetector 55, which is not one of the four photodetectors 51, 52, 53, and 54 which form the photodetector assembly, is used for synchronization between a scanning system and sensor signals.
To increase precision in the measurement of 6-DOF motion with the apparatus of FIG. 5, it is preferable to increase the size of the photodetector assembly. Thus, there is difficulty in measuring the motion of a small object with precision. In addition, the rate of obtaining measurement data is limited by the scanning speed of the scanning system, and thus the ability to measure the motion of an object that moves fast is limited by the scanning speed.
To solve the above problems of the conventional art, it is a first object of the present invention to provide an apparatus for measuring 6 degree-of-freedom (DOF) motion of an object, which can easily and precisely measure high-speed displacement of a small object using a multidirectional reflector.
It is a second object of the present invention to provide a structurally simple swing arm type optical system which uses the 6-DOF motion measuring apparatus to measure 6-DOF motion of a slider in a hard disc drive (HDD) and can accurately measure the dynamic characteristics of the slider in tracking and searching tracks.
To achieve the first object of the present invention, there is provided an apparatus for measuring 6 degree-of-freedom (DOF) motion of an object using a laser beam emitted from a laser source, the apparatus comprising: a multidirectional reflector having at least three reflecting sides by which the laser beam is slit and reflected in three directions, the multidirectional reflector being provided to the object whose motion is to be measured; three position-sensitive detectors for receiving three sub-laser beams reflected from the multidirectional reflector; and a controller for calculating the 6-DOF motion of the multidirectional reflector using the intensity distributions of the three sub-laser beams received by the three position sensitive detectors assuming that the laser beam before reflection has a Gaussian intensity distribution.
It is preferable that the laser beam from the laser source tracks the apex of the multidirectional reflector at which the three reflecting sides meet. It is preferable that the laser source can move in two dimensions such that the laser beam emitted from the laser source tracks the apex of the multidirectional reflector at which the three reflecting sides meet. It is preferable that the controller receives electric signals from the position-sensitive detectors, and analyzes the intensity distributions of the three sub-laser beams received by the position-sensitive detectors to determine whether or not the intensity distributions of the three sub-light beams are the same. It is preferable that the controller adjusts the location of the laser source if the intensity distributions of the three sub-light beams are not the same.
To achieve the second object of the present invention, there is provided a swing arm type optical system using a laser beam emitted from a laser beam scanner to measure 6 degree-of-freedom (DOF) motion of a slider in a hard disc drive (HDD), the swing arm type optical system comprising: a multidirectional reflector having three reflecting sides on which the laser beam is simultaneously incident, the multidirectional reflector being mounted on or adjacent to the slider, wherein the relative positions of the slider and the multidirectional reflector are fixed; at least one optical path forming reflector for adjusting the traveling path of the laser beam scanned from the laser beam scanner such that the laser beam is incident on the apex of the multidirectional reflector at which the three reflecting sides meet; three position-sensitive detectors disposed in the optical paths of three sub-laser beams reflected from the multidirectional reflector; a controller for measuring the 6-DOF motion of the multidirectional reflector by analyzing the intensity distributions of the three sub-laser beams received by the three position sensitive detectors assuming that the laser beam before reflection has a Gaussian intensity distribution; and a plurality of swing arms which support the slider and along which the traveling path of the laser beam is formed.
It is preferable that the rear ends of the plurality of the swing arms are connected to a pivot, and the plurality of swing arms pivot around the pivot. It is preferable that the plurality of swing arms comprise an upper swing arm and a lower swing arm, a through hole is formed at the front end of the upper swing arm, and the laser beam travels along the direction of the upper swing arm and is incident on the apex of the multidirectional reflector through the through hole.
It is preferable that the plurality of swing arms comprise an upper swing arm and a lower swing arm, the upper swing arm is formed as a rigid body, the lower swing arm includes a suspension and a flexure which are joined together, and the slider is mount on the bottom of the flexure.
It is preferable that the optical path forming reflector comprises a first reflector mounted on the top of the pivot about which the upper and lower swing arms pivot, and a second reflector mounted at the through hole of the upper swing arm; and the laser beam emitted from the laser beam scanner is reflected by the first and second reflectors and is incident on the apex of the multidirectional reflector.
It is preferable that the first and second reflectors have a 45-degree sloping side, the 45-degree sloping sides of the first and second reflectors face each other, the laser beam emitted from the laser beam scanner is reflected by the 45-degree sloping side of the first reflector toward the 45-degree sloping side of the second reflector, and the laser beam reflected by the 45-degree sloping side of the first reflector is reflected by the 45-degree sloping side of the second reflector such that the reflected laser beam is incident on the apex of the multidirectional reflector through the through hole.
It is preferable that the first and second reflectors have a 45-degree sloping side, the 45-degree sloping sides of the first and second reflectors are parallel sloping down toward the front end of the upper swing arm, the laser beam scanned from the laser beam scanner is reflected by the 45-degree sloping side of the first reflector toward the 45-degree sloping side of the second reflector, and the laser beam reflected by the 45-degree sloping side of the first reflector is reflected by the 45-degree sloping side of the second reflector such that the reflected laser beam is incident on the apex of the multidirectional reflector through the through hole.